Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 60-6N recorded around the circumference of each servo track. Each servo sector 6i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6i further comprises groups of servo bursts 14 (e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations. A position error signal (PES) is generated by reading the servo bursts 14, wherein the PES represents a measured position of the head relative to a centerline of a target servo track. A servo controller processes the PES to generate a control signal applied to a head actuator (e.g., a voice coil motor) in order to actuate the head radially over the disk in a direction that reduces the PES.
The data sectors are accessed indirectly using logical block addresses (LBAs) mapped to physical block addresses (PBAs) representing the physical location of each data sector. This indirect accessing facilitates mapping out defective data sectors during manufacturing as well as while the disk drive is deployed in the field. Access commands (read/write) received from the host include LBAs which the disk drive maps to corresponding PBAs using any suitable mapping technique.
The LBA to PBA mapping may also facilitate log structured writes wherein at least part of the disk is written as a circular buffer. For example, the circular buffer may be written from an outer diameter track toward an inner diameter track, and then circle back to the outer diameter track. Data is written to the head of the circular buffer such that the LBAs of new write commands are mapped to the PBAs of the corresponding data sectors. When the same LBA is written by the host, the data is written to a new PBA at the head of the circular buffer and the old PBA is marked invalid so that it may be overwritten. During a garbage collection operation, valid PBAs previously written in the circular buffer may be relocated to the head of the circular buffer so that the old PBAs may be overwritten. In one embodiment, the tracks are written in a shingled manner such that a previously written track is partially overwritten, thereby increasing the overall capacity of the disk drive.